三水鋁石(gibbsite, Al(OH)3)是工業上生產α-Al2O3的主要原料。其相轉換至α-Al2O3的過程複雜。起始粒徑、熱處理條件、氣氛等因素均會影響其相轉換路徑並經由不同過渡相。一般就粒徑的影響而言，已知有兩條相轉換路徑：粒徑<1μm的粒子循著gibbsite →χ-Al2O3 →κ-Al2O3 →α-Al2O3路徑，而粗粒(>1μm ) gibbsite則另會同時發生第二條路徑：gibbsite → boehmite →γ-Al2O3 →θ-Al2O3 →α-Al2O3。由於gibbsite至α-Al2O3相轉換屬於假形相轉換(pseudomorphic transformation)，過去在這方面的觀察並不完整。本研究觀察gibbsite的假形特性與相轉換過渡相的關係。另外探討奈米級粒徑的gibbsite及χ-Al2O3，其相轉換路徑是否另有改變及其原因。研究分為兩部分：（1）觀察gibbsite結晶在相轉換過程，過渡相的微結構晶簇(drusy aggregates)及晶癖(crystal habit)發育特性，了解其與假形相轉換的關係。（2）觀察奈米級粒徑gibbsite及χ-Al2O3粉體的相轉換過程。
研究結果發現，gibbsite在相轉換過程的晶癖發育特殊。其六方最密堆積(hexagonal close packing)晶格(001)面在相變為χ-及κ-Al2O3過程循原有層狀晶格發育為片狀層層堆疊結構。厚度由χ-相的10 nm成長至κ-相的40~100 nm。此時片片間形成間距，且此片面均仍為(001)面。在轉為α-相之後則發育為蠕蟲狀(vermicular)結構，單晶截面直徑約200 nm。同時層狀結構漸不明顯。在此片體平面上的晶簇變化，χ-Al2O3片體具有尺寸約10 nm的微晶微孔，在轉變為κ-相後此微孔消失，κ-Al2O3並具有沿{110}的雙晶現象。Gibbsite→χ-Al2O3→κ-Al2O3→α-Al2O3具有的假形相轉換特性，可能係因粒體內部呈現規則的晶簇外型改變，可平衡因比重升高而需產生的體積收縮，使得粒體外型維持不變。
當gibbsite粒徑<200 nm時，相轉換將循著gibbsite→χ-Al2O3 →α-Al2O3路徑，原本相轉換過程中的κ-Al2O3過渡相將不出現。以不同粒徑χ-Al2O3作觀察，當χ-Al2O3粒徑小至~40 nm以下，也有相同現象。χ→α-Al2O3相轉換的α-Al2O3初生成晶徑約為30 nm，可能為一對一的相轉換。平均粒徑~40 nm的χ-Al2O3粉末也具有最低的α-相生成活化能。

Gibbsite (Al(OH)3) is the major raw material of industrial production of α-alumina, the routes of transformation from gibbsite to α-Al2O3 were affected by the particle size, heating rate, pressure and water vapour. It was known that two routes appeared as a result of particle size effect, gibbsite →χ-Al2O3 →κ-Al2O3 →α-Al2O3 take place when particle size <1μm. For coarse particle (>1μm), a second route take place parallelly, gibbsite → boehmite →γ-Al2O3 →θ-Al2O3 →α-Al2O3. Gibbsite to α-Al2O3 transformation displaying characteristics of pseudomorphic transformation, but the study so far was not sufficient. In this study, the relationships between transition phases and pseudomorphic characteristic during transformation of gibbsite were examined. Altered transformation route indused by size effects were also discussed. The investigation including two portions: (i) developments of drusy aggregates and crystal habit characteristic during transformation of gibbsite crystal, and the relationship with pseudomorphic transformation. (ii) phase transformation of nano-sized gibbsite and χ-Al2O3 powders.
It was found that a particular crystal habit development during phase transformation of gibbsite. The layered lattice of hexagonal close packing of oxygen ions along (001) was continued to develop platelet-stacking structures after transforming to χ- and κ-Al2O3. The thickness of χ-platelet was 10 nm and then grew up to 40~100 nm for κ-platelet. The (001) basal face of platelets were remained and stacked with an interval space with each other. Vermicular structure appeared after transforming to α-phase and eliminated the lamellar texture gradually, the cross section of single crystal was about 200 nm. The developments of drusy aggregates on the plane varied with the formation of transition phases, χ-platelet possessed nanocrystallites and pores of about 10 nm in size, the pores were eliminated from the κ-phase formation, a 120° twinning operation of κ-Al2O3 domains along {110} twin plane can be easily observed. The mechanism of pseudomorphic transformation of gibbsite→χ-Al2O3→κ-Al2O3→α-Al2O3 may result from a self-assembly evolution of nano-scaled crystal habit occurring in the interior of gibbsite crystals, that balanced the volume reduction due to increase in density during the transformation, thus remaining the externals of particle.
Gibbsite→χ-Al2O3 →α-Al2O3 transformation route occurred with skippingκ-Al2O3 when particle size of gibbsite less than 200 nm. Same phenomenon occurred in χ-Al2O3 powders, direct transformation from χ-Al2O3 to α-Al2O3 took place when particle size of χ-Al2O3 less then 40 nm. The formation of α-Al2O3 nucleus from χ→α-Al2O3 transformation may carry out from one χ-Al2O3 particle to one α-Al2O3 crystallite, the crystal size of α-Al2O3 nucleus was about 30 nm. χ-Al2O3 powder with mean particle size about 40 nm possessed the lowest α-Al2O3 formation energy.

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